In recent years, supercritical fluid chromatography (SFC), which is characterized by high resolution and short analysis time and capability of analyzing nonvolatile and thermally unstable chemical species, has attracted attention. One example of such SFC is disclosed in Analytical Chemistry, Vol. 54, No. 4, April 1982, pp. 736-740.
FIG. 1 is a schematic drawing showing the essential structure of an SFC apparatus. This apparatus comprises a pressure vessel 1 filled with gas, a liquid-delivering pump 2 equipped with a chiller, an injector 3, a microcolumn 4, an oven 5, an UV detector 6, and a constant-pressure release valve 7.
In the operation of this apparatus, the gas inside the pressure vessel 1 is pressurized to about or above its critical point by the pump 2 having the chiller. The pressurized gas, or mobile phase, is introduced into the column 4 at a constant flow rate. If the gas is CO.sub.2, it is pressurized above the critical point of CO.sub.2, i.e., above 73 atm., by the pump 2 and then forced into the column 4. The temperature of the inside of the column 4 is kept above the critical point of CO.sub.2, or above 31.degree. C., by the oven 5 and, therefore, a supercritical fluid of CO.sub.2 is formed inside the column. Under this condition, a sample is injected from the injector 4. The sample is then dissolved in the supercritical fluid, developed, and separated in the column. The separated eluates are successively sent to the UV detector 6 and detected. Subsequently, they are discharged to the atmosphere via the constant-pressure release valve 7. Where the mobile phase is an organic solvent that is in liquid phase at ordinary temperature, such as hexane, it is not necessary to liquify the mobile phase and so the pump 2 is not required to have a chiller. Where a sample which is not readily dissolved in a supercritical fluid of CO.sub.2 is analyzed, CO.sub.2 gas to which a modifier such as alcohol has been added is used as the mobile phase. The constant-pressure release valve 7 is installed to maintain the inside of the flow passage system at a certain pressure exceeding the critical point.
Since the open end, or the discharge side, of the constant-pressure release valve is at the atmospheric pressure, while the mobile phase kept at a high pressure is passing into the valve, the pressure rapidly decreases, and the supercritical condition is no longer retained. Consequently, the sample which was dissolved in the supercritical fluid is separated out and deposited in the valve. The sample deposits in the discharge passage of the release valve and clogs it up. This varies the pressure inside the flow passage system of the apparatus, thus making the base line of the resulting chromatogram unstable. In the worst case, the discharge passage of the release valve is completely clogged up with the deposited sample, and the pressure inside the flow passage system increases. Then, the reproducibility and the separability deteriorate, whereby the sample cannot be analyzed.